The present invention relates broadly to a control system and method for an automatic transmission adapted to shift the transmission without vacuum assistance.
Vehicle transmissions of the automatic variety generally comprise a torque converter and a compound planetary gear set controlled by clutches and bands which provide the frictional elements necessary to the shift the transmission from one gear to another. The torque converter supplements the gear set by multiplying engine torque. The torque converter and clutches couple the engine to the planetary gears through oil pressure to typically provide up to three forward speeds as well as a reverse speed.
The transmission is generally controlled by a hydraulic system. Hydraulic pressure is supplied by a transmission oil pump which typically is engine driven and an associated line system. Main line pressure from the pump to the transmission is controlled by a pressure regulator valve in the pump and by a vacuum modulator connected to the engine to provide an engine vacuum signal. The main line pressure is the activating power of the transmission and is supplied to apply or release bands and to control clutches, servos, sprags, valves, governors, and the like which, in turn, control the gear changes which upshift or downshift the transmission. For example, whenever the engine of the vehicle is running at idle speed with the selector lever or prindle in either the "park" or the "neutral" position, no line pressure is directed to any of the mechanical units in the transmission. Accordingly, all clutches and bands are released. When, however, the selector lever is placed in the "drive" position and the vehicle commences movement into first speed, line pressure is directed to, for example, a forward clutch where it acts upon the clutch piston to apply the forward clutch and thereby place the transmission in low gear. The pressure regulator controls line pressure such that the torque requirements of the transmission clutches are met and the proper shift spacing is obtained at all throttle openings.
The vacuum modulator controls main line pressure via a valve which supplies a modulator pressure to the hydraulic system that is proportional to engine torque and to vehicle speed. The modulator pressure is used to vary the shift points of the transmission in correspondence with the opening of a throttle and to raise hydraulic line pressure proportional to engine torque. The modulator valve is biased by an evacuated bellows, a diaphragm and a spring so arranged that the bellows and the spring apply a force to the valve to increase modulator pressure. Engine vacuum and fluid pressure from a governor provided in proportion to vehicle speed act on the modulator valve in a manner which opposes the bellows to thereby decrease modulator pressure as the speed of the vehicle increases. A more detailed description of vacuum modulators, governors, and of automatic transmissions in general may be had by reference to U.S. Pat. Nos. 3,106,104, 3,165,008, 4,291,595, 4,621,544, and to Motor's Automatic Transmission Manual (4th ed. 1971), the disclosures of which are expressly incorporated herein by reference.
However, to properly control the line pressure, the modulator pressure must vary in proportion to the torque input to the transmission. Inasmuch as torque input to the clutches is a product of engine torque and converter ratio, modulator pressure must compensate for changes in either or both. Accordingly, modulator pressure is regulated by engine vacuum which is an indication of engine torque and carburetor opening. Modulator pressure thus is provided to decrease with an increase in vehicle speed to compensate for a change in converter torque ratio.
In view of the foregoing, it may be appreciated that the vacuum modulator as controlled by the governor and by engine vacuum has heretofore been an integral part of the automatic transmission. However, in vehicles such as trucks and the like powered by combustion engines of the diesel variety, no engine vacuum is available with which to drive a modulator. Although such vehicles may be provided with a vacuum pump to develop the necessary pressure differential, vacuum pumps and the associated lines and cables involve a significant expense and require considerable maintenance to assure operability. Moreover, it is often desirable to retrofit a truck or the like having a worn or blown gasoline engine with an engine of the more economical diesel variety. Heretofore, such a retrofit has required the addition of the aforementioned vacuum pump and its associated lines, or the replacement of the transmission. Both these alternatives, however, are objectionable from an economic standpoint. Accordingly, a need has existed which, as of yet, has be left unmet for a control system adapted to shift or actuate the gears of an automatic transmission without vacuum assistance.